为什么 lock(this) {...} 不好? [英] Why is lock(this) {...} bad?
问题描述
public class SomeObject
{
public void SomeOperation()
{
lock(this)
{
//Access instance variables
}
}
}
是如果实例可以公开访问的问题".我想知道为什么?是不是因为锁的持有时间超过了必要的时间?还是另有隐情?
is "a problem if the instance can be accessed publicly". I'm wondering why? Is it because the lock will be held longer than necessary? Or is there some more insidious reason?
推荐答案
在 lock 语句中使用 this 是不好的形式，因为通常你无法控制还有谁可能锁定了那个对象.
It is bad form to use this in lock statements because it is generally out of your control who else might be locking on that object.
为了正确地计划并行操作，应该特别注意考虑可能的死锁情况，并且具有未知数量的锁入口点会阻碍这一点.例如，任何拥有对象引用的人都可以在对象设计者/创建者不知道的情况下锁定它.这增加了多线程解决方案的复杂性，并可能影响其正确性.
In order to properly plan parallel operations, special care should be taken to consider possible deadlock situations, and having an unknown number of lock entry points hinders this. For example, any one with a reference to the object can lock on it without the object designer/creator knowing about it. This increases the complexity of multi-threaded solutions and might affect their correctness.
私有字段通常是更好的选择，因为编译器会对其实施访问限制，并且会封装锁定机制.使用 this 会向公众公开部分锁定实现，从而违反了封装.除非已记录在案，否则您是否将获得对 this 的锁定也不清楚.即便如此，依靠文档来预防问题也不是最佳选择.
A private field is usually a better option as the compiler will enforce access restrictions to it, and it will encapsulate the locking mechanism. Using this violates encapsulation by exposing part of your locking implementation to the public. It is also not clear that you will be acquiring a lock on this unless it has been documented. Even then, relying on documentation to prevent a problem is sub-optimal.
最后，有一个常见的误解，即 lock(this) 实际上修改了作为参数传递的对象，并以某种方式使其只读或不可访问.这是错误.作为参数传递给lock 的对象仅用作键.如果那把钥匙已经被锁住了，就不能开锁；否则，允许锁定.
Finally, there is the common misconception that lock(this) actually modifies the object passed as a parameter, and in some way makes it read-only or inaccessible. This is false. The object passed as a parameter to lock merely serves as a key. If a lock is already being held on that key, the lock cannot be made; otherwise, the lock is allowed.
这就是为什么在 lock 语句中使用字符串作为键是不好的，因为它们是不可变的，并且可以在应用程序的各个部分共享/访问.您应该改用私有变量，Object 实例会很好.
This is why it's bad to use strings as the keys in lock statements, since they are immutable and are shared/accessible across parts of the application. You should use a private variable instead, an Object instance will do nicely.
运行以下 C# 代码作为示例.
Run the following C# code as an example.
public class Person
{
public int Age { get; set; }
public string Name { get; set; }
public void LockThis()
{
lock (this)
{
System.Threading.Thread.Sleep(10000);
}
}
}
class Program
{
static void Main(string[] args)
{
var nancy = new Person {Name = "Nancy Drew", Age = 15};
var a = new Thread(nancy.LockThis);
a.Start();
var b = new Thread(Timewarp);
b.Start(nancy);
Thread.Sleep(10);
var anotherNancy = new Person { Name = "Nancy Drew", Age = 50 };
var c = new Thread(NameChange);
c.Start(anotherNancy);
a.Join();
Console.ReadLine();
}
static void Timewarp(object subject)
{
var person = subject as Person;
if (person == null) throw new ArgumentNullException("subject");
// A lock does not make the object read-only.
lock (person.Name)
{
while (person.Age <= 23)
{
// There will be a lock on 'person' due to the LockThis method running in another thread
if (Monitor.TryEnter(person, 10) == false)
{
Console.WriteLine("'this' person is locked!");
}
else Monitor.Exit(person);
person.Age++;
if(person.Age == 18)
{
// Changing the 'person.Name' value doesn't change the lock...
person.Name = "Nancy Smith";
}
Console.WriteLine("{0} is {1} years old.", person.Name, person.Age);
}
}
}
static void NameChange(object subject)
{
var person = subject as Person;
if (person == null) throw new ArgumentNullException("subject");
// You should avoid locking on strings, since they are immutable.
if (Monitor.TryEnter(person.Name, 30) == false)
{
Console.WriteLine("Failed to obtain lock on 50 year old Nancy, because Timewarp(object) locked on string "Nancy Drew".");
}
else Monitor.Exit(person.Name);
if (Monitor.TryEnter("Nancy Drew", 30) == false)
{
Console.WriteLine("Failed to obtain lock using 'Nancy Drew' literal, locked by 'person.Name' since both are the same object thanks to inlining!");
}
else Monitor.Exit("Nancy Drew");
if (Monitor.TryEnter(person.Name, 10000))
{
string oldName = person.Name;
person.Name = "Nancy Callahan";
Console.WriteLine("Name changed from '{0}' to '{1}'.", oldName, person.Name);
}
else Monitor.Exit(person.Name);
}
}
控制台输出
'this' person is locked!
Nancy Drew is 16 years old.
'this' person is locked!
Nancy Drew is 17 years old.
Failed to obtain lock on 50 year old Nancy, because Timewarp(object) locked on string "Nancy Drew".
'this' person is locked!
Nancy Smith is 18 years old.
'this' person is locked!
Nancy Smith is 19 years old.
'this' person is locked!
Nancy Smith is 20 years old.
Failed to obtain lock using 'Nancy Drew' literal, locked by 'person.Name' since both are the same object thanks to inlining!
'this' person is locked!
Nancy Smith is 21 years old.
'this' person is locked!
Nancy Smith is 22 years old.
'this' person is locked!
Nancy Smith is 23 years old.
'this' person is locked!
Nancy Smith is 24 years old.
Name changed from 'Nancy Drew' to 'Nancy Callahan'.
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